1. Field of the Invention
This invention relates generally to medical diagnostic ultrasound methods and apparatus and more particularly to an apodization parameter generator for a medical diagnostic ultrasound apparatus.
2. Description of the Related Art
Medical diagnostic ultrasound is the imaging of internal areas of a patient's body using ultrasound energy. Ultrasound energy is sound wave energy having a frequency greater than approximately 20 kHz. To generate ultrasound energy, electronic signals are input to a transducer which converts the electrical signals into ultrasound signals. Ultrasound signals, typically of 2 MHz to 10 MHz, are transmitted into a patient's body where they are in-part absorbed, dispersed, refracted and reflected. Reflected ultrasound signals are received at transducer elements which convert the reflected ultrasound signals back into electronic signals. A transmitter coupled to the transducer is controlled to form a transmit beam-pattern of ultrasound signals. A receiver coupled to the transducer is controlled to form a receive beam-pattern of ultrasound signals. A final ultrasound beam-pattern, to the first order, is a product of the transmit beam-pattern and the receive beam-pattern. The final beam-pattern typically is processed to analyze echo, Doppler and flow information and obtain an image of the patient's encountered anatomy (e.g., tissue, flow, Doppler).
Apodization is one of several beam-forming control parameters. Beam-forming parameters are used to control the final beam-pattern and enable meaningful scanning of a patient's anatomy. The beam-forming parameters include: aperture, apodization, focus and steering. Aperture is a control of the number of active transducer elements along the transducer array's azimuth or elevation. Apodization is a voltage weighting profile of the active elements. Focus is a time delay profile of such weighting. Steering is a control of focus "depth" point(s) along the azimuth or elevation.
Samples of ultrasound signals are processed to define a beam-pattern. Typically a beam-pattern has a main lobe (i.e., main beam) and multiple side lobes (i.e., side beams). Side lobes are ultrasound phase alignments at other than the desired steering angle (i.e., at other than the main lobe). Preferably, the main lobe is at a much higher decibel (db) level than the side lobes, so that echoes from patient anatomy corresponding to side lobes are substantially attenuated relative to echoes corresponding to the main lobe. A function of apodization control is to shape the beam-pattern. Specifically, apodization control is used to reduce the side lobes in the beam-pattern. In doing so, the main lobe becomes wider.
Apodization control is also used for reducing other extraneous acoustic signals, such as grating lobes, from the beam-pattern. Grating lobes are unwanted redundant beams. Typically, they are not as large as a main lobe, but larger than an average side lobe. Grating lobes result from the geometry of a multi-element transducer array. Specifically, when the spacing between transducer elements exceeds one-half of a wavelength at the operating frequency of the transducer array, grating lobes occur. Apodization control can be used to reduce grating lobes.
Side lobes and grating lobes are undesirable as they can result in side lobe artifacts or grating lobe artifacts. Side lobe artifacts are incorrect presentations of image information caused by sampling the side lobes. Sampling and grating lobe artifacts are incorrect presentations of image information caused by sampling the side or grating lobes. Grating lobes, for example, introduce errors and noise into the imaging process. Although the main lobe is directed at a target spatial area at a given instant in time, the contributions to the return echo include reflections caused by the grating lobes of different points. As a result, the image is smeared by the several contributions. Undesirable side lobe or grating lobe artifacts include duplication of image features, (e.g., duplicate fetal bones appearing like amniotic bands or sheets).
Effectively selecting apodization parameters reduces side lobes and grating lobes and corresponding artifacts. In generating a final beam-pattern, a window function is implemented. The beam-forming control parameters are input to the window function to achieve the final beam-pattern. The desired apodization parameters typically vary according to the window function and the size of the aperture. The window function typically changes according to the operating mode of the ultrasound system. For example, a different window may be used for B-mode scanning than for C-mode scanning. Also, a different window may be used for sonographic ultrasound than for Doppler ultrasound. Because ultrasound scans for different operating modes or different types of ultrasound often are interleaved, the apodization parameters may need to change regularly. Also, as the aperture may change dynamically while a patient is examined, the apodization parameters may need to change dynamically.
Previously, apodization parameter generation in medical ultrasound systems has been a processing-intensive method performed in a processing subsystem. Apodization parameters have been calculated, then downloaded to the transmitter or receiver beam-former in real-time. As a result, apodization parameter generation introduces significant processing overhead to the ultrasound system. There is a desire to reduce such overhead and free up processing time for other performance improving functions or to enable other more processing-intensive ultrasound applications. Accordingly, there is need of a more efficient method for generating apodization parameters.
Previously, an apodization parameter look-up table has been implemented in radar applications. According to such application, the table is a "straight" look-up table, including a separate set of parameters for each possible algorithm perturbation. Implementation of such a look-up table for medical ultrasound applications would require an excessive amount of random access memory (RAM), and consume substantial amounts of power and board space. The look-up table for an m-element transducer array, according to the radar implementation approach, would include approximately m! entries. Such a table would consume more power and more board space than the beam-former to receive the parameters or than a processing subsystem for calculating such parameters.
Accordingly, there is need for a more efficient apparatus and method for generating apodization parameters in a medical diagnostic ultrasound system.